non-interactive version of "pmbootstrap init"

The -y option in pmbootstrap currently confirms all yes/no questions.

Make it all questions in pmbootstrap init answer with the default value, so pmbootstrap -y init can be used to go through init non-interactively.

Currently we use yes "" | pmbootstrap init in CI scripts, but that's of course not as nice.

EDIT: rewritten to use -y instead of --auto